Hybrid drives for work machines include a combustion engine, a generator driven by the combustion engine, a charge storage unit, and an electric engine. The propulsion of the work machine or the drive by components driven by the work machine are carried out by a drive train, in which the drive torque of the combustion engine and the electric engine are superimposed by a summation gear, e.g., a planetary gear, or act jointly on a shaft. Such hybrid drives can be operated in different operating modes, such as (a) a load point shifting mode, in which the generator charges the charge storage unit or the electric engine supplied by the charge storage unit makes available an additional drive torque for the drive train and improves the efficiency of the combustion engine; (b) a recuperation mode, in which the electric engine is used as a generator and converts the kinetic energy of the work machine into electric energy for the charging of the charge storage unit; and (c) a boost mode, in which the combustion engine and the electric engine supplied by the charge storage unit are operated with maximum power, so as to overcome short-term load peaks. The switch between the operating modes takes place as a function of the momentary speed of the combustion engine and the momentary load torque.
In the load point shifting mode, there is a division of the drive power into the power made available by the combustion engine and the power made available by the charge storage unit. Therefore, such hybrid drives make available another level of freedom in comparison to conventional drives, in which only the power of the combustion engine can be changed. For the specification of the torque of the combustion engine and the torque of the electric engine or for the division of a power being made available between the combustion engine and the electric engine, different types of regulators are used in the state of the art, in particular proportional-integral regulators or heuristic regulators. Such so-called power or torque distribution regulators must observe additional boundary conditions, such as an effort to maintain the theoretical value of a specific charge in the charge storage unit, and to optimize, in the sense of an optimizing of the total effectiveness.
The transition behavior between the operating modes can prove to be problematic with the power or torque distribution regulators used in the state of the art. If, for example, the recuperation mode is active, the charging state of the charge storage unit is increased by the recuperated kinetic energy. If, subsequently, the load point shifting mode is active, the regulator attempts to, once again, compensate for the charging increase. In this way, the charge in the charge storage unit is again reduced, but at the expense of a more unfavorable efficiency of the combustion engine. However, with a subsequent boost, then, the recuperated charge in the charge storage unit is missing.
In the state of the art, operating strategies have been described which deal with a load point shift, a recuperation, and a boost, and are based on heuristics or optimizing methods. With heuristics, low hardware demands are advantageous, but at the expense of an often low consumption reduction and a high parameterization expense. With optimizing methods, a higher reduction in consumption can be attained, but with complex hardware demands.